The present invention relates to a printing head of an impact dot printer, and more particularly, to a printing head capable of smoothly moving reciprocating needles whose tips are arranged in two vertical columns, thereby preventing damage from the needles and ribbon jamming.
In an impact dot printer the needles having tips are arranged in a zigzag array of two vertical columns, along with solenoids for driving the needles are disposed in an annular array. It is known to arrange the solenoids, associated with needles of one needle column, on a corresponding semicircle of the annular array in the same order as that of the needles in that needle column. In this case, two solenoids, associated with two laterally adjacent needles at the upper ends of the two vertical columns forming the zigzag array, are disposed to be adjacent to each other on the annular array. Also, two solenoids associated with two laterally adjacent needles at the lower end of the zigzag array are adjacent to each other on the annular array.
During the printing process, one or more solenoids are energized to generate magnetic flux thereby forwardly moving one or more needles, each coupled to an associated armature arranged to be pivoted upon generation of the magnetic flux. Further, the printer operates to cause each solenoid to be deenergized at a desired timing so that the magnetic flux is extinguished when the needle reaches its forwardmost position, thereby permitting the needle to be restored by the spring force of an associated spring. Moreover, in the conventional printer, the solenoids are so designed that the direction of the magnetic flux generated by one of adjacent solenoids is opposite to that of magnetic flux generated by the other solenoid, for reduction of magnetic interference between the adjacent solenoids.
The conventional printer, which is arranged to generate, by each adjacent solenoid pair, magnetic fluxes whose directions are opposite to each other. When two solenoids corresponding to laterally adjacent needles at the lower end of the zigzag array of two vertical columns are alternately energized for printing an underline, for instance, the timing of the solenoid operation is altered due to the effect of the magnetic flux generated by another solenoid. Consequently, reciprocal movement of the needle concerned is adversely affected. For example, if part of the magnetic flux generated by one of the solenoids acts on another solenoid when the magnetic flux of the latter solenoid is extinguished, the backward movement of the needle associated with the latter solenoid is prolonged. In this case, the tip of the needle can be caught by the ribbon tape to cause damage to the needle or ribbon from jamming.
A dot printer, which is so designed that the direction of magnetic fluxes generated by some of a plurality of electromagnets which cooperate to form a common magnetic flux path is opposite to that of magnetic fluxes generated by remaining electromagnets, is described in Japanese Patent Publication No. 58-35475. However, this publication merely contemplates reducing a magnetic flux amount in the common magnetic flux path. The cross section area of the magnetic flux path is reduced, thereby providing a compact printing head. This printer still entails the aforesaid drawback.